1. Field of the Invention
The present invention relates to an electron beam printing apparatus which is capable of high-quality pattern exposure free from multiple exposure when printing a fine pattern on a circuit printing base.
2. Description of the Related Art
Avoidance of multiple exposure is essential for printing a fine pattern on a mask or a wafer with accuracy by scanning an electron beam in the field of the production of a semiconductor.
In designing a mask pattern, a building block system is generally adopted in order to facilitate the design. This is a method of constructing a layout by combining various blocks which are defined in advance. The final layout drawing which is a combination of separate layers is therefore a combination of about 2 to 4 rectangles or polygonals in most cases.
If the patterns of the layout drawing are exposed to an electron beam one by one, multiple exposure is produced in a considerable number of regions. As a result, a non-uniformity of amount of radiation is produced, in other words, the portions of over-exposure are produced in the same drawing, thereby greatly deviating the actual pattern dimension from the dimension in the design.
Especially, when a strict pattern accuracy is required, multiple exposure is the problem to be eliminated.
Such multiple exposure is also the problem in printing a pattern on a circuit printing base with a photoresist applied thereto by scanning an electron beam as in wafer exposure.
To explain this, the way a printing pattern CAD outputs a printing pattern will first be described.
The most general system for exposing a pattern on a circuit printing base is, at present, a film exposing system using ultraviolet rays. In this system, the draft is often drawn on art work film by a drafting machine called a photo plotter or laser plotter. When a printing pattern data output from the printing pattern CAD is inputted to the drafting machine, the data is converted into data of a format which is suitable for drafting, thereby executing drafting on the film. A format called Gerber format is now predominant as the format of printing pattern data outputted from the printing pattern CAD and inputted to the drafting machine. This format fundamentally consists of the aperture size which corresponds to the pattern width, the coordinates of the starting point (Xs, Ys) and the end point (Xe, Ye) of each segment of the printing pattern, and a series codes for indicating whether or not the respective segments are to be exposed.
FIG. 8(A) shows a pattern of one segment represented by data in a Gerber format. The trajectory of a circle having a diameter of Da, which is the designated aperture size Da corresponding to the pattern width, and having linearly moved from the starting point (Xs, Ys) to the end point (Xe, Ye) is represented by a pattern segment.
FIG. 8(B) shows the state in which a pattern for one segment is scanned by an electron beam. In the case of scanning by an electron beam, the pattern having a line width exceeding the diameter of the beam spot is scanned as an aggregate of a plurality of vectors parallel to the center line of the pattern. This is concept of printing is called a vector scanning system.
FIG. 8(C) shows an example of a pattern of winding obtained by connecting patterns of segments. If a pattern for one segment is faithfully drawn by vector scanning as shown in FIG. 8(B), double exposure is produced in the areas represented by the hatches in FIG. 8(C).
Such multiple exposure does not matter in the case of printing by raster scanning. In the raster scanning system, only the portions to be exposed are successively irradiated for printing by horizontal scanning as in the case of television scanning. In the case of printing by raster scanning, the bit map image of the patterns is generally formed in the memory. The bit map image is a binary graphic produced by resolving the printing pattern into dots for the respective bits which correspond to the beam spot, and classifying into the pattern portions as bit [1] and the other portions as bit [0]. This information is written into the memory. At the time of printing, the dot data are successively read out of the memory and the beam is radiated to the portions having bit [1]and not radiated to the portions having bit [0]. Consequently, even if there are portions which are apparently subjected to multiple exposure in the design, no multiple exposure is produced in the actual work.
In the case of printing by raster scanning, however, deflection scanning on the entire surface is required irrespective of the ratio of the areas under pattern occupation in the base.
In contrast, in the case of printing by vector scanning, since only the pattern portions are scanned, the effective printing time is naturally shortened. The ratio of the areas under pattern occupation in ordinary circuit printing base is as small as 20 to 30%, so that the effect on the reduction in the effective printing time is large.
Development of technique for securing the pattern accuracy in the printing by vector scanning while avoiding multiple exposure is therefore important from the point of view of throughput.
Some of the conventional electron beam exposure apparatii for exposing a wafer by using a mask are provided with a function of detecting a portion subjected to multiple exposure, as disclosed in, for example, Japanese Patent Laid-Open No. 56769/1979. The contents are summarized as follows:
In designing a mask pattern used for exposure of a wafer, various blocks defined in advance are combined for constructing a layout The final layout drawing is a combination of rectangles or polygonals. Rectangular areas which are in contact with the vertices of the respective rectangles or polygonals and contain the respective figures are set. If the rectangular areas of two figures do not overlap with each other, these two figures are regarded as including no overlapping portion and printed as they are. On the other hand, if they overlap with each other, whether or not there is an overlapping portion as oblique figures is examined in a rotating coordinate system. In this way, since the electron beam exposure apparatus is provided with a function of automatically detecting the overlap of patterns, the processing for eliminating overlap is greatly reduced.
The above-described related art pertains to a system for detecting overlap between figures, but a method for avoiding overlap, if any, is not concretely described.
In the case of directly printing a circuit on a base by vector scanning, operation for examining at what position and to what extent one pattern overlaps with another is also necessary.
This operation is not easy. For example, it may seem that the existence of an overlap at the connecting point of the pattern segments (I) and (II) is easily found in FIG. 8(C), but it is actually difficult because there is no graphic continuity in the arrangement of the pattern data.
To state this more concretely, if the Gerber data were arranged in the right order such as the order of the patterns (I), (II), (III) . . . , the relationship of the connection between the patterns would be comparatively easily examined. In most of the actual operations, however, they are not arranged in the right order. For example, the horizontal patterns (II, IV) are first designated, the vertical patterns are next designated and the oblique patterns (I, III) are finally designated. In order to examine the relationship of connection between the pattern segments, it is therefore necessary to first read the Gerber data on all pattern segments on the entire surface and to retrieve the pattern segment which is connected to the pattern segment (I). Similar operations are required in the other pattern segments. Such operations are generally executed in by software in a computer, and the higher the density the pattern has, the larger the amount of time is required for processing. In addition, after the relationship between the respective pattern segments and the overlapping areas are comprehended, it is further necessary to divide the pattern drawing so as not to produce multiple exposure (overlap), and to develop the respective divided drawings into vector data which are necessary for printing by vector scanning.
If the operations of retrieving the relationship between the respective pattern segments and the overlapping areas over the entire surface of the base, dividing the drawing on the basis of the retrieved data so as not to produce an overlap and developing the respective divided drawings into vector data in order to avoid multiple exposure, as described above, are executed by processings of a computer, the amount of data is so enormous that it takes a long processing time. If it is executed by a hardware processing using an electron circuit, the circuit structure becomes too complicated for practical use.